New assessment procedure and technologies for monuments

The Trajan Arch in Benevento is one of the most important and well preserved monuments of the roman empire, although along its history of about two thousand years it suffered many damages due to important events, such as earthquakes, wars, and, last but not least, partial reconstructions, and restorations (Fig.1). Such important structural changes interested the monument in different periods: most of them on its top section, the Attic, probably already during its or just after its construction.

The preservation of the Trajan Arch and its historical value for future generations requires, therefore, a dedicated program of preservation, with conservation actions and safeguarding measures, programmed and optimized along the time [1][2]. It is, therefore, very important the acquisition of an effective knowledge of its present health status and of its evolution along the years. This knowledge can be obtained with an optimized design and implementation of a continuous and distributed monitoring of all its important structural elements in connection with a careful analysis based on an adaptive structural FEM (Finite Element Method) dynamic model of the arch.

 

This procedure can provide information both on the dynamic and static loads sources (seismic noise, anthropic noise, wind noise, etc.) which affect the monument and on the effects on its sculptures and structural elements, such as to provide reliable description and clarification of the existing damage patterns, like the ones in the Fornice.

The key element of the monitoring system is a folded pendulum, a mechanical monolithic sensor belonging to the class of large band – high sensitivity position and/or acceleration monolithic sensors based on the Watt’s linkage architecture developed at the University of Salerno (UNISA Folded Pendulum). By prof. Fabrizio Barone.

The UNISA Folded pendulum is an innovative version of Watt’s linkage and a mechanical oscillator of remarkable properties, capable to resonate at very low frequency, still keeping compact size and weight [3][4], In fact, taking advantage of the local gravitational acceleration, allows the design of compact, light and stable mechanical oscillators, that is the basis of high quality and sensitivity mechanical seismometers and/or accelerometers. The modern techniques of mechanical machining using precision milling ad electro-discharge machining (EDM) allows the implementation of very compact and light sensors with typical natural resonance frequency (100 mHz), large measurement band (10⁻⁷ Hz – 10³ Hz) and sensitivities (<10⁻¹² m/Hz^1/2) in the low frequency region of the seismic spectrum, still keeping light weight (< 200 g), small size (< 10 cm side) and large immunity to environmental noises.

The mechanical transfer function, and in particular, the natural resonance frequency, can be changed by applying external forces or by introducing calibration masses or suitable external forces, changing, in this way, also the sensitivity curve [3-8].

Fig. 3 shows the mechanics of a medium size (8 cm side, 0.3 kg weight) UNISA uniaxial monolithic accelerometer for low frequency large band measurement implemented in Ergal (AL7075-T6), anodized for long term environmental applications and designed to host the most common readouts (shadow meters, optical levers, LVDTs, fiber channels, interferometers), used for the preliminary tests on the Trajan Arch. Fig. 4. shows, instead, a triaxial sensor obtained with three monolithic UNISA Folded Pendulums oriented along the three Cartesian axes (x, y, z), that allows the implementation of an effective and stable tridimensional measurement point.

Finally, Table 1 shows the main characteristics of typical mechanical sensors based on the UNISA Folded Pendulum architecture.

 

Characteristic	Properties
Band	0.1    uHz < B < 1 kHz
Sensitivity	10-15 m/Hz1/2 < S <   10-6 m/Hz1/2
Directivity	D > 104
Reson. Frequency	50     mHz < fo < 1 kHz

Table 1. UNISA Folded Pendulum Main Characteristics.

Two tests were performed on top of the Trajan Arch with a limited number of sensors, the first one in July 2015 (few hours) necessary for the definition of preliminary requirements on sensors and monitoring system, the second one in late September 2015 (4 days) using a simultaneous acquisition of 4 sensors, necessary to understand the optimal placement of the sensors and of the data acquisition system on the Attic. Preliminary measurement were performed also at the base of the Arch in order to understand the noise floor level, to correlate the measurements of the sensors on the top of the Arch.

These measurements highlight the need of an effective and continuous monitoring also of wind speed and direction. In fact, the effect of the wind and traffic is very relevant for the dynamic behavior of the Trajan Arch. The first test was performed during a windy day in Benevento and this effect is visible in Fig. 7, that shows the displacement spectral density of three sensors positioned on the top of the Arch (the first two sensors on the Arch center, the third one on the lateral plinth) in the band 100 mHz – 100 Hz). It is evident the strong effect of the wind on the whole band, in particular in the central part of the Arch.

The mechanical properties of the stone blocks on site, taking into account the variability and the arrangement of the blocks was taken on the basis of a bibliographic analysis and data described in the report of the prof. Salvatore D’Agostino (1991), “..It is constituted by a core wall in limestone boulders covered with marble slabs pario…”.

In particular the following reference mechanical characteristics have been at first considered (Table 2):

Table 2. Mechanical Properties

Weight (kN/m2)	Modulus of Elasticity E (GPa)
26,00	25,00

For the study the Arch has been modeled by linear finite elements by using the “SAP2000 Advanced v17.3.0” softwaredeveloped by CSI (Computers and Structures Inc.) di Berkeley.

The FEM model involves the use of macro-elements solid, able to describe three-dimensional stress states. The Arch has been subdivided in several FEM macro-elements and subsequently automatic discretized by an appropriate meshing, which allowed to carry out more refined analysis in correspondence of section variations.

At the base the model has fixed constraints, being the dynamic recorded response modest.

The first three modal shapes respectively show transversal, longitudinal and rotation motion of the top side. The upper modal shapes involve deformations in the walls of the Attic.  Fig.5 describes the comparison between the evaluated main modal periods (frequencies) and the Fourier Spectrum of the recorded signals of the first test. It is possible to observe that the numerical evaluated modal periods do not fit the processed field test data with regard both the transverse (Y) and longitudinal (X) behavior. The observed shift on the frequencies between the modal and the signal analyses shows a stiffer behavior of the Arch FEM model. The result appears consistent, observing that the Arch was built as a superposition of blocks and that the external Attic walls were realized by coupling a thin stone wall not rigidly linked, while the FEM model considers the Arch as unique stone block.

Fig.5 Modal Shape

The figure shows a quite good agreement between the numerical modal analysis and the Fourier Spectrum of the acquired signals. In correspondence of the first mode (transversal displacement of all the top side of the Arch) both the signals recorded in the center (red) and lateral (blue) of the external wall present resonance peaks. For the second shape mode a resonance peak could be read only on the recorded longitudinal signals (magenta) and, last, for the third shape mode only the lateral recorded signal (blue) present a resonance peak. The fifth modal shape presents resonance peaks on both central (red) and lateral (blue) recorded signals. In this case, there is a greater shift from the recorded signals results and the numerical ones. The Fourier Spectrum shows also a peak on the central recorded signal around 9 Hz that could not be explained by the linear analysis carried out. The latter observations could be explained by considering local behavior due to the real composition of the external walls in correspondence of the Arch. Finally, it is interesting to observe that equivalent Young modulus is considerably lower of that describing the blocks material.

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REFERENCES

1. M. Petzet, “Principles of Preservation. An Introduction to Int. Charters for Cons. and Restor. 40 Years after the Venice Charter”, ICOMOS, München, pp.7-29.ISBN 3-87490-676-0, 2004.
2. E.Tortorella, I. Marino, M. N. Khanlou, U. Dorka, L. Petti, “Seismic response control of rigid block systems by using tendon system: The case of greek columns”. Joint Conf. Seismic Protection of Cultural Heritage Pag.149-161,WCCE-ECCE- TCCE, 2011.
3. F. Barone, G. Giordano, F. Acernese, “Low frequency folded pendulum with high mechanical quality factor in vertical configuration, and vertical seismic sensor utilizing such a folded pendulum”,(PCT), WO 2012/147112 (2012), Patent Number: IT 1405600, AU 2012247104, Europe, Japan, Russia, USA, Canada pending.
4. F. Barone, R. De Feo, G. Giordano, A. Mammone, L. Petti, L. TomayA New Strategy of Monitoring in Cultural Heritage Preservation: the Trajan Arch in Benevento as a Case of Study.
5. L. Petti, G. Giannattasio, M. De Iuliis Small Scale Experimental Testing on base isolation and tuned mass damper combined control strategy

6. L. Petti , M. Modesti , I. Marino Sperimentazione di nuove tipologie di ancoraggio di rinforzi in FRP nel restauro statico di costruzioni monumentali in muratura